JP7374817B2 - Drive shaft and its manufacturing method - Google Patents

Description

The present invention relates to a drive shaft for transmitting driving force generated by a driving force generation mechanism to wheels of an automobile, and a method for manufacturing the drive shaft.

A drive shaft for an automobile needs to be lightweight and have excellent rigidity at the end where a constant velocity joint or the like is provided. In order to meet this demand, for example, Patent Document 1 proposes configuring the drive shaft as a joined body of a hollow tubular body and a solid stub shaft. That is, weight reduction is achieved by making the central portion of the drive shaft excluding the ends a hollow tube. In addition, high rigidity is achieved by using a solid stub shaft at the end of the drive shaft.

For example, friction welding is employed as a method for joining the hollow tube and the solid stub shaft. When performing friction welding, a first annular wall provided in an annular shape at the end of the hollow tubular body, and an annular wall provided at the end of the solid stub shaft so as to have an annular shape substantially the same as the first annular wall. and the second annular portion are friction welded together. As a result, the hollow tube body and the solid stub shaft are joined via the friction welding portion formed between the first annular wall and the second annular wall.

Japanese Patent Application Publication No. 2008-87003

A burr is formed on the friction welding portion. The burrs formed on the outer peripheral surfaces of each of the first annular part and the second annular part can be removed by machining or the like from the outside of the drive shaft. However, the burrs formed on the inner circumferential surfaces of each of the first annular part and the second annular part cannot be removed from the outside of the drive shaft and remain in the product of the drive shaft. Depending on the shape of the burr, there is a concern that the proximal end of the burr may become a stress concentration area, reducing the fatigue strength of the drive shaft.

The present invention was made to solve the above problems, and provides a drive shaft with excellent fatigue strength and torsional strength, and a method for manufacturing the same.

One aspect of the present invention includes a first annular wall provided annularly at the end of a hollow tube made of medium carbon steel, and a first annular wall provided annularly at the end of a solid stub shaft made of medium carbon steel. A drive shaft in which two annular walls are axially joined via a friction welding portion, each of the first annular wall and the second annular wall having an outer diameter of 30 to 50 mm and a wall thickness. 3 to 5 mm, and burrs are formed in the friction welding portion, rising from a first inner wall surface that is an inner wall surface of the first annular wall and a second inner wall surface that is an inner wall surface of the second annular wall. , the burr has a burr bottom portion that is a base end rising from the first inner wall surface, and an orthogonal portion that extends in a direction perpendicular to the first inner wall surface, and has a connecting radius that is a radius of curvature of the burr bottom portion. r is 0.5 mm or more, and the root radius R, which is the minimum radius of curvature of the portion of the burr between the burr bottom and the orthogonal part, is 0.5 mm or more, and the burr bottom with respect to the first inner wall surface The burr bottom angle θ, which is the inclination angle of be.

Another aspect of the present invention is a first annular wall provided annularly at the end of a hollow tube made of medium carbon steel, and a first annular wall provided annularly at the end of a solid stub shaft made of medium carbon steel. A method for manufacturing a drive shaft in which a drive shaft is obtained by joining the solid stub shaft with a second annular wall via a friction welding part, the method comprising: a forging step of obtaining the solid stub shaft by cold forging, and an outer diameter of 30 to 50 mm. and a friction welding step of friction welding the first annular wall and the second annular wall having a wall thickness of 3 to 5 mm, and in the friction welding step, the inner wall surface of the first annular wall A burr rising from a first inner wall surface and a second inner wall surface which is an inner wall surface of the second annular wall is such that the burr bottom, which is a base end rising from the first inner wall surface, is perpendicular to the first inner wall surface. and an orthogonal part extending in the direction, the connecting radius r, which is the radius of curvature of the burr bottom, is 0.5 mm or more, and the minimum radius of curvature of the part of the burr between the burr bottom and the orthogonal part is A certain root radius R is 0.5 mm or more, a burr bottom angle θ, which is an inclination angle of the burr bottom with respect to the first inner wall surface, is 40° or less, and the axis of the portion between the burr bottom and the orthogonal part The friction welding portion is formed so that the length L of the burr inclined portion along the direction is 0.2 to 5 mm.

In this drive shaft, the shape of the burr formed on the friction welding portion is set as described above. Therefore, stress concentration on the burr can be suppressed, and the fatigue strength of the drive shaft can be improved. Moreover, it becomes possible to improve the torsional strength of the drive shaft by properly joining the hollow tube body and the solid stub shaft through the friction welding portion.

FIG. 2 is a partially cross-sectional side view along the longitudinal direction of the drive shaft according to the embodiment of the present invention. FIG. 2 is a schematic explanatory diagram of a main part explaining the shape of a burr. It is a micrograph of the 1st annular wall and the 2nd annular wall in a friction welding part. It is a micrograph of the hollow tube body (first annular wall) in the friction welding part. It is a micrograph of the solid stub shaft (second annular wall) in the friction welding part. 1 is a schematic flowchart of a method for manufacturing a drive shaft according to an embodiment of the present invention. FIG. 2 is a schematic sectional view of the main parts of the hollow tube and the solid stub shaft before friction welding. FIG. 3 is a schematic cross-sectional view of the main parts of the hollow tube body and solid stub shaft after friction welding. It is a graph showing the relationship between the connecting radius r and root radius R of the burr and the maximum stress concentration coefficient. It is a graph showing the relationship between the connecting radius r and root radius R of the burr and the breaking torque.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a drive shaft and a method for manufacturing the same according to the present invention will be described in detail with reference to the accompanying drawings. In addition, in the following figures, the same reference numerals are given to the component which has the same or similar function and effect, and the repeated description may be omitted.

As shown in FIG. 1, in the drive shaft 10, solid stub shafts 14 are integrated at both ends of a hollow tube body 12 in the axial direction. First annular walls 16 are provided at both ends of the hollow tube body 12, respectively. A second annular wall 18 is provided at the end of the solid stub shaft 14 . The second annular wall 18 is formed to have substantially the same shape as the first annular wall 16.

Each of the first annular wall 16 and the second annular wall 18 has an outer diameter of 30 to 50 mm and a wall thickness of 3 to 5 mm. The first annular wall 16 and the second annular wall 18 are friction-welded in the axial direction. As a result, the hollow tube body 12 and the solid stub shaft 14 are joined via the friction welding portion 20 formed on the first annular wall 16 and the second annular wall 18. At one end and the other end of the hollow tubular body 12 in the axial direction, friction welding portions 20 are formed in substantially the same way.

As shown in FIG. 2, the friction welding portion 20 has burrs rising from a first inner wall surface 22a, which is the inner wall surface of the first annular wall 16, and a second inner wall surface 24a, which is the inner wall surface of the second annular wall 18. 26 is formed. Specifically, the burr 26 includes a first curved portion 28 that rises from the first inner wall surface 22a and curves toward the side opposite to the second inner wall surface 24a in the axial direction, and a first curved portion 28 that rises from the second inner wall surface 24a. It has a first inner wall surface 22a in the axial direction and a second curved portion 30 that curves toward the opposite side.

The first curved portion 28 has a burr bottom portion 32 that is a base end rising from the first inner wall surface 22a, and an orthogonal portion 34 that extends in a direction orthogonal to the first inner wall surface 22a. It is preferable that the radius of curvature of the burr bottom 32, the connecting radius r1 (connecting radius r), is 0.5 to 3 mm. Furthermore, the minimum radius of curvature of the portion of the burr 26 between the burr bottom 32 and the orthogonal portion 34, ie, the root radius R1 (root radius R), is preferably 0.5 to 2.5 mm. Further, the burr bottom angle θ1 (burr bottom angle θ), which is the inclination angle of the burr bottom portion 32 with respect to the first inner wall surface 22a, is 40° or less. Furthermore, the burr slope portion length L1 (burr slope length L), which is the length along the axial direction of the portion between the burr bottom portion 32 and the orthogonal portion 34, is 0.2 to 5 mm.

The second curved portion 30 has a burr bottom portion 36 that is a base end rising from the second inner wall surface 24a, and an orthogonal portion 38 extending in a direction orthogonal to the second inner wall surface 24a. The connecting radius r2, the root radius R2, the burr bottom angle θ2, and the burr slope length L2 in the second curved portion 30 are not particularly limited, but the connecting radius r1 and the root radius R1 in the first curved portion 28 are not particularly limited. , burr bottom angle θ1, and burr slope length L1 are preferably set substantially the same.

A valley bottom portion 40 is formed between the first curved portion 28 and the second curved portion 30. An imaginary line passing through this valley bottom 40 in the radial direction of the first annular wall 16 and the second annular wall 18 is defined as an imaginary center line I. As shown in FIG. 3, in the friction welding portion 20, the axial end portion 18a of the second annular wall 18 extends 1 to 30 μm from the virtual center line I toward the first annular wall 16. As shown in FIG.

In this embodiment, the hollow tube body 12 in FIG. 1 is made of medium carbon steel. Suitable examples of the medium carbon steel include C of 0.43 to 0.47%, Si of 0.30% or less, Mn of 0.60 to 0.90%, and 0.010% or less. Contains P, 0.020% or less S, 0.1% or less Cu, 0.1% or less Ni, 0.05% or less Cr, 0.02 to 0.04% Al, and the balance is Examples include Fe and unavoidable impurities. Further, the grain size of this medium carbon steel is #5 to #9 when expressed as a grain size number based on ASTM E112.

The solid stub shaft 14 of FIG. 1 is also made of medium carbon steel. Suitable examples of the medium carbon steel include 0.45 to 0.51% C, 0.25% or less Si, 0.30 to 0.50% Mn, and 0.010% or less by weight. P, 0.008-0.020% S, 0.1% or less Cu, 0.1% or less Ni, 0.1-0.2% Cr, at least 0.05-0.25% Mo, 0.03 to 0.08% Nb, any one of 0.01 to 0.05% Ti, 0.02 to 0.04% Al, 10 to 30 ppm B, Examples include those in which the balance is Fe and unavoidable impurities.

That is, the medium carbon steel that is the material of the solid stub shaft 14 contains more Mo, Nb, and Ti than the medium carbon steel that is the material of the hollow tube body 12. Further, the grain size of the medium carbon steel forming the solid stub shaft 14 is #9 to #11 when expressed as a grain size number based on ASTM E112.

The frames in FIGS. 4 and 5 represent grain boundaries. As a result of determining the crystal grain size of the hollow tube body 12 (first annular wall 16) in the friction welding part 20 based on the magnification and frame dimensions of the micrograph in FIG. It was 5 to #9. As a result of determining the crystal grain size of the solid stub shaft 14 (second annular wall 18) in the friction welding part 20 based on the magnification and frame dimensions of the micrograph in FIG. It was 10 to #12. From this, in both the solid stub shaft 14 and the hollow tube body 12 in FIG. 1, the crystal grains are refined in the friction welding portion 20.

When analyzing the vicinity of the grain boundaries of the crystal grains in the friction welding part 20, it was found that Mo ₂ C, NbC, and TiC are present as precipitated particles at the grain boundaries in the metal structure of the second annular wall 18. Admitted. From this, it is inferred that Mo ₂ C, NbC, and TiC suppress the growth of crystal grains.

Next, a method for manufacturing the drive shaft 10 according to the present embodiment will be described based on the schematic flow shown in FIG. 6. This manufacturing method includes a forging step S1 for obtaining the solid stub shaft 14, a friction welding step S2 for friction welding the hollow tube body 12 and the solid stub shaft 14, an annealing step S3, and a quenching step S4.

As a material for obtaining the solid stub shaft 14, medium carbon steel having the above composition ratio is preferable. This steel material made of medium carbon steel is subjected to rolling at a temperature of 850° C. or lower. Due to rolling in such a temperature range, distortion remains in the steel material. Further, transformation from austenite to ferrite occurs, and ferrite is precipitated from the unevenness where distortion remains. As a result of the above, a soft structure with fine crystal grains and easy molding is formed.

After that, the steel material is subjected to spheroidizing annealing. At that time, for example, after holding the temperature at 720 to 760°C for a predetermined time, slow cooling may be performed at a cooling rate of 0.5°C/min or less to 600°C. This progresses the spheroidization of cementite (Fe ₃ C), resulting in a structure in which a large amount of relatively soft ferrite exists.

Next, as a forging step S1, cold forging is performed on the steel material after spheroidizing annealing. By performing cold forging in this manner, coarsening of crystal grains can be avoided. In other words, the crystal grains are maintained in a fine state before and after cold forging. Moreover, since the steel material is soft, it can be easily formed into the shape of the solid stub shaft 14 even by cold forging.

Next, a friction welding step S2 is performed. Note that the friction welding step S2 can be performed in accordance with JIS Z 3607, for example. In the friction welding step S2, for example, the solid stub shaft 14 shown in FIG. 7 is held in a rotating holder (not shown) and rotated. Further, the hollow tube body 12 in FIG. 7 is held in a propulsion holder (not shown). After the rotation of the solid stub shaft 14 reaches a predetermined rotation speed (for example, 2.5 to 10 m/sec), the hollow tube body 12 is brought close to the solid stub shaft 14 coaxially with the propulsion holder. let By bringing the first annular wall 16 and the second annular wall 18 into pressure contact in this manner, frictional pressure is applied to the joint interface between them, and the joint interface generates heat due to frictional heat. The friction pressure at this time is, for example, 20 to 60 MPa.

When the joining interface, whose temperature has increased due to the frictional heat described above, reaches a desired softened state, the rotation of the solid stub shaft 14 by the rotary holder is stopped. Then, at the timing when the solid stub shaft 14 stops rotating, the hollow tube body 12 is propelled toward the solid stub shaft 14 by the propulsion holder. This applies an upset pressure of, for example, 50 to 200 MPa to the bonding interface.

As a result, as shown in FIG. 8, the first annular wall 16 and the second annular wall 18 are solid-phase joined via the friction welding part 20. At this time, a portion of the first annular wall 16 and the second annular wall 18 are discharged from the joint interface toward the inner and outer circumferential sides of the friction welding portion 20 by plastic flow. As a result, the friction welding portion 20 has a structure that rises from the first inner wall surface 22a and first outer wall surface 22b of the first annular wall 16, and the second inner wall surface 24a and second outer wall surface 24b of the second annular wall 18. A burr 26 is formed.

In this friction welding step S2, the above-mentioned rotation speed, friction pressure and Set friction welding conditions such as upset pressure. In this case, it is estimated that the internal temperature of the first annular portion and the second annular portion near the bonding interface reaches 700°C. As a result, at least one of Mo ₂ C, NbC, and TiC is precipitated, particularly at the grain boundaries in the metal structure of the second annular wall 18 .

Mo ₂ C, NbC, and TiC precipitate at grain boundaries, and have a pinning effect to prevent coarsening of crystals and improve the strength of grain boundaries. As a result, the first annular wall 16 and the second annular wall 18 in the friction welding portion 20 are hardened, and the strength and yield strength at high temperatures of, for example, about 600° C. can be improved.

In addition, the second annular wall 18 becomes harder than the first annular wall 16 due to the above-mentioned precipitation occurring particularly on the second annular wall 18 . Therefore, the first annular wall 16, which is more deformable than the second annular wall 18, deforms so as to wrap around the second annular wall 18. As a result, in the friction welding part 20, as shown in FIG. The wall 16 and the second annular wall 18 are firmly joined.

In the above friction welding step S2, the second annular walls 18 of the solid stub shaft 14 are friction welded to the first annular walls 16 at both ends of the hollow tube body 12 in the axial direction. Note that the friction welding step S2 may be performed by holding the hollow tubular body 12 in a rotating holder and holding the solid stub shaft 14 in a propulsion holder.

Next, an annealing step S3 is performed. That is, the drive shaft 10 is heated to a predetermined temperature. This annealing step S3 removes distortion generated during friction welding and promotes recrystallization. By recrystallization, the crystal grains become a fine structure of about 20 μm. Further, NbC, VC, Mo ₂ C, etc. are precipitated at the grain boundaries also in the annealing step S3. Due to the above-mentioned grain refinement and precipitation of carbides at grain boundaries, the friction welded portion 20 exhibits excellent strength. Note that the annealing temperature is preferably 650 to 720°C and the holding time is preferably 30 to 90 minutes.

Next, the drive shaft 10 is subjected to a predetermined machining process to remove, for example, the burr 26 formed on the outer peripheral side of the friction welding portion 20 in FIG. 8 . That is, the burrs 26 rising from each of the first outer wall surface 22b of the first annular wall 16 and the second outer wall surface 24b of the second annular wall 18 are removed. As a result, as shown in FIGS. 1 and 2, the friction welding portion 20 has a first curved portion rising from each of the first inner wall surface 22a of the first annular wall 16 and the second inner wall surface 24a of the second annular wall 18. 28 and a second curved portion 30.

By setting the friction welding conditions in the friction welding step S2 as described above, as shown in FIG. 2, the connecting radius r1 of the first curved portion 28 becomes 0.5 to 3 mm, and the root radius R1 becomes 0.5 to 3 mm. 2.5 mm, the burr bottom angle θ1 is 40° or less, and the burr slope length L1 is 0.2 to 5 mm.

Next, the drive shaft 10 that has been shaped as described above is subjected to a hardening step S4 to obtain the drive shaft 10 shown in FIG. 1. In the hardening step S4, it is preferable to perform induction hardening because of various advantages such as excellent thermal efficiency. In induction hardening, the drive shaft 10 is subjected to induction heating and then cooled. During this cooling, since the burr 26 of the friction welding portion 20 has the above-described shape, it is possible to suppress heat from being trapped in the burr 26 . This can suppress variations in hardness and quench cracks in the friction welded portion 20.

Further, in this hardening step S4, the entire drive shaft 10 can be hardened. Here, in the solid stub shaft 14 made of medium carbon steel with the composition as described above, quenching can proceed more easily and a sufficient hardened layer can be formed compared to other medium carbon steels.

In this way, the medium carbon steel that is the material of the solid stub shaft 14 has excellent hardenability. Therefore, even if the hollow tube 12 is thinner than the solid stub shaft 14 and hardened under conditions that form a hardened layer of sufficient depth in the hollow tube 12, the solid stub shaft The hardened layer depth of 14 can be ensured. That is, coarsening of crystal grains in the structure of the hardened hollow tube body 12 is avoided. Therefore, the strength of the hollow tube body 12 can be improved by quenching while avoiding coarsening of the crystal grains.

Therefore, there is no need to increase the outer diameter or wall thickness of the hollow tube 12 without hardening it to ensure strength. By this amount, the weight of the drive shaft 10 can be reduced. Further, since the hollow tube body 12 is not exposed to excessive heat treatment, occurrence of quench cracks in the drive shaft 10 is avoided.

Moreover, in the drive shaft 10, the crystal grains of the friction welding portion 20 are fine, and carbides are precipitated at the grain boundaries. This carbide provides a so-called particle dispersion strengthening effect. For the above reasons, the friction welded portion 20 has excellent strength and toughness.

Furthermore, since the hollow tubular body 12 is lightweight, the drive shaft 10 can be made lightweight. That is, in this embodiment, even with the drive shaft 10 in which the hollow tubular body 12 and the solid stub shaft 14 are joined, it is possible to ensure sufficient strength while reducing the weight.

From the above, in the drive shaft 10 and the manufacturing method thereof according to the present embodiment, the connecting radius r1 is 0.5 mm or more and the root radius R1 is 0.5 mm or more with respect to the first inner wall surface 22a of the friction welding part 20. A burr 26 is formed in which the burr bottom angle θ1 is 40° or less and the burr slope length L1 is 0.2 to 5 mm.

The friction welding conditions for forming the burr 26 having the above shape can be determined as follows. That is, for example, test friction welding conditions are determined, and the friction welded portion 20 is formed under these test friction welding conditions. The friction welding portion 20 is observed under a microscope to measure the joint radius r1, root radius R1, burr bottom angle θ1, and burr slope length L1. From these measured values and the test friction welding conditions, friction welding conditions in which the connecting radius r1, root radius R1, burr bottom angle θ1, and burr slope length L1 fall within the above ranges are determined.

Here, the results of a copper plating stress measurement method performed to determine the relationship between the respective sizes of the connecting radius r1 and the root radius R1 and the maximum stress concentration factor of the burr 26 are shown in FIG. In this copper plating stress measuring method, first, the friction welding conditions for forming a plurality of friction welding test specimens having different sizes of the connecting radius r1 or root radius R1 were determined as described above. For a plurality of friction welding test pieces produced based on these friction welding conditions, the maximum stress concentration coefficient was measured using the plating stress measurement method.

As shown in FIG. 9, it was found that when each of the connecting radius r1 and the root radius R1 was 0.5 mm or more, the maximum stress concentration coefficient was less than the reference value. When the maximum stress concentration factor is less than or equal to the reference value, it can be said that stress concentration on the burr 26 can be sufficiently suppressed when the drive shaft 10 is used. Therefore, by setting the connecting radius r1 to 0.5 mm or more and the root radius R1 to 0.5 mm or more, stress concentration on the burr 26 can be suppressed and the fatigue strength of the drive shaft 10 can be improved. .

Next, FIG. 10 shows the results of a simulation (Computer Aided Engineering (CAE)) performed to determine the relationship between the respective sizes of the connecting radius r1 and the root radius R1 and the fracture torque of the drive shaft 10. From FIG. 10, it was found that when each of the connecting radius r1 and the root radius R1 is 0.5 mm or more, the breaking torque exceeds the reference value of the breaking torque required for the drive shaft 10. Therefore, the torsional strength of the drive shaft 10 can be favorably improved by setting the connecting radius r1 to 0.5 mm or more and the root radius R1 to 0.5 mm or more.

In this drive shaft 10, by setting the burr bottom angle θ1 to 40° or less, it becomes easy to make each of the connecting radius r1 and the root radius R1 0.5 mm or more. Further, by setting the length L1 of the burr slope portion to 0.2 mm or more, it is possible to make the bead portion formed by solidifying the bonding interface softened by frictional heat in the friction welding step S2 to a sufficient size. As a result, the connecting radius r1 can be easily set to 0.5 mm or more, and the bonding strength in the friction welding portion 20 can be increased.

As mentioned above, since each of the first annular wall 16 and the second annular wall 18 has an outer diameter of 30 to 50 mm and a wall thickness of 3 to 5 mm, the welding margin in friction welding is about 5 mm. good. Therefore, by setting the length L1 of the burr inclined portion to 5 mm or less, it is possible to avoid an unnecessarily increased joining margin in friction welding. Furthermore, since the ratio of the materials of the first annular part and the second annular part used to form the friction welding part 20 can be reduced, the material yield can be improved.

Further, by setting the length L1 of the burr inclined portion to 5 mm or less, it is possible to avoid the burr 26 from becoming larger than necessary. Therefore, it is possible to prevent the burr 26 from breaking due to its own weight. Further, during cooling after high-frequency heating, it becomes easier to suppress heat from being trapped in the burr 26, and it is possible to suppress variations in hardness and quench cracks in the friction welded portion 20. As a result, the bonding strength in the friction welding portion 20 can be increased.

In the drive shaft 10 according to the above embodiment, the joint radius r1 is 3 mm or less, and the root radius R1 is 2.5 mm or less. Further, in the friction welding step S2 of the method for manufacturing the drive shaft 10 according to the above embodiment, the friction welding portion 20 is formed in which the connecting radius r1 is 3 mm or less and the root radius R1 is 2.5 mm or less.

By setting the joint radius r1 to 3 mm or less, it is possible to avoid the burr 26 from becoming larger than necessary, so that the bonding strength in the friction welding portion 20 can be increased. Further, by setting the root radius R1 to 2.5 mm or less, the connecting radius r1 can easily be set to 0.5 mm or more, and the bonding strength in the friction welding portion 20 can be increased.

In the drive shaft 10 according to the embodiment described above, the burr 26 includes a first curved portion 28 that rises from the first inner wall surface 22a and curves toward the side opposite to the second inner wall surface 24a in the axial direction, and a second inner wall surface 24a. It has a second curved part 30 that rises from the wall surface 24a and curves toward the opposite side to the first inner wall surface 22a in the axial direction, and is formed between the first curved part 28 and the second curved part 30. When an imaginary line passing through the valley bottom portion 40 along the radial direction of the first annular wall 16 and the second annular wall 18 is an imaginary center line I, in the friction welding portion 20, the axial end of the second annular wall 18 The portion 18a is set to extend from the virtual center line I to the first annular wall 16 side by 1 to 30 μm. In this case, as described above, it is possible to form the friction welding portion 20 in which the first annular wall 16 and the second annular wall 18 are firmly joined.

In the friction welding portion 20 of the drive shaft 10 according to the above embodiment, at least one of NbC, Mo ₂ C, and TiC is precipitated at the grain boundaries in the metal structure of the second annular wall 18. .

Further, in the drive shaft 10 according to the above embodiment, the medium carbon steel that is the material of the solid stub shaft 14 has a weight ratio of C: 0.45 to 0.51%, Si: 0.20% or less, Mn: 0.30 to 0.50%, P: 0.010% or less, S: 0.008 to 0.020%, Cu: 0.1% or less, Ni: 0.1% or less, Cr: 0. 1 to 0.2%, Mo: 0.05 to 0.25%, Nb: 0.03 to 0.08%, Ti: 0.01 to 0.05% (however, at least any of Mo, Nb, and Ti (1), Al: 0.02 to 0.04%, B: 10 to 30 ppm, the remainder being Fe and inevitable impurities, and the particle size number was #9 to #11.

Further, in the drive shaft 10 according to the above embodiment, the medium carbon steel that is the material of the hollow tube body 12 has a weight ratio of C: 0.43 to 0.47%, Si: 0.30% or less, Mn: 0.60 to 0.90%, P: 0.010% or less, S: 0.020% or less, Cu: 0.1% or less, Ni: 0.1% or less, Cr: 0.05% or less , Al: 0.02 to 0.04%, the balance being Fe and unavoidable impurities, and the particle size number was #5 to #9.

Furthermore, in the method for manufacturing the drive shaft 10 according to the above embodiment, after the friction welding step S2, the drive shaft 10 is annealed under the conditions of a holding temperature of 650 to 720° C. and a holding time of 30 to 90 minutes. It includes an annealing process S3 and a quenching process S4 in which the drive shaft 10 after the annealing is hardened, and NbC, Mo ₂ C, and TiC are precipitated at the grain boundaries in the metal structure of the solid stub shaft 14. I decided to let him do it.

In these cases, the precipitated Mo ₂ C, NbC, and TiC precipitate at the grain boundaries and inhibit the growth of the crystal grains with a pinning effect, thereby improving the strength of the grain boundaries. As a result, the bonding strength in the friction welding portion 20 can be improved. Therefore, when the drive shaft 10 is subjected to a tensile test, the hollow tubular body 12, not the friction welded portion 20, breaks.

Further, in the hardening step S4, the entire drive shaft 10 can be hardened. Therefore, even with the drive shaft 10 in which the hollow tubular body 12 and the solid stub shaft 14 are joined, it is possible to ensure sufficient strength while reducing the weight.

The present invention is not particularly limited to the embodiments described above, and various modifications can be made without departing from the gist thereof.

DESCRIPTION OF SYMBOLS 10... Drive shaft 12... Hollow tube body 14... Solid stub shaft 16... First annular wall 18... Second annular wall 18a... End portion 20... Friction pressure welding part 22a... First inner wall surface 24a... Second inner wall surface 26 ...Burr 28...First curved part 30...Second curved part 32...Burr bottom 34...Orthogonal part 40...Valley bottom I...Virtual center line L, L1...Burr slope part length r, r1...Connection radius R, R1...Root R θ, θ1…burr bottom angle

Claims (9)

A first annular wall provided annularly at the end of a hollow tube made of medium carbon steel and a second annular wall provided annularly at the end of a solid stub shaft made of medium carbon steel are friction welded. A drive shaft axially joined via a portion,
Each of the first annular wall and the second annular wall has an outer diameter of 30 to 50 mm and a wall thickness of 3 to 5 mm,
In the friction welding portion, burrs are formed that rise from a first inner wall surface that is an inner wall surface of the first annular wall and a second inner wall surface that is an inner wall surface of the second annular wall,
The burr has a burr bottom part that is a base end rising from the first inner wall surface, and an orthogonal part along a direction orthogonal to the first inner wall surface,
The connecting radius r, which is the radius of curvature of the bottom of the burr, is 0.5 mm or more,
The root radius R, which is the minimum radius of curvature of the portion of the burr between the burr bottom and the orthogonal portion, is 0.5 mm or more,
A burr bottom angle θ, which is an inclination angle of the burr bottom with respect to the first inner wall surface, is 40° or less,
A drive shaft, wherein a length L of a burr inclined portion, which is a length along the axial direction of a portion between the burr bottom and the orthogonal portion, is 0.2 to 5 mm. The drive shaft according to claim 1,
The connecting radius r is 3 mm or less,
The drive shaft, wherein the root radius R is 2.5 mm or less. The drive shaft according to claim 1 or 2,
The burr includes a first curved portion that rises from the first inner wall surface and curves toward a side opposite to the second inner wall surface in the axial direction, and a first curved portion that rises from the second inner wall surface and curves toward the opposite side of the second inner wall surface in the axial direction. a second curved portion that curves toward the opposite side of the first inner wall surface;
When a virtual line passing through a valley bottom formed between the first curved part and the second curved part along the radial direction of the first annular wall and the second annular wall is defined as a virtual center line,
In the friction welding portion, the axial end of the second annular wall extends 1 to 30 μm from the virtual center line toward the first annular wall. The drive shaft according to any one of claims 1 to 3,
In the drive shaft, at least one of NbC, Mo ₂ C, and TiC is precipitated at the grain boundaries in the metal structure of the second annular wall in the friction welding portion. The drive shaft according to any one of claims 1 to 4,
The medium carbon steel that is the material of the solid stub shaft has a weight percentage of C: 0.45 to 0.51%, Si: 0.20% or less, Mn: 0.30 to 0.50%, and P: 0.010% or less, S: 0.008 to 0.020%, Cu: 0.1% or less, Ni: 0.1% or less, Cr: 0.1 to 0.2%, Mo: 0.05 to 0.25%, Nb: 0.03 to 0.08%, Ti: 0.01 to 0.05% (however, at least one of Mo, Nb, and Ti), Al: 0.02 to 0.08%. 04%, B: 10 to 30 ppm, the balance being Fe and unavoidable impurities, and the particle size number is #9 to #11. The drive shaft according to any one of claims 1 to 5,
The medium carbon steel that is the material of the hollow tube has a weight percentage of C: 0.43 to 0.47%, Si: 0.30% or less, Mn: 0.60 to 0.90%, and P: Contains 0.010% or less, S: 0.020% or less, Cu: 0.1% or less, Ni: 0.1% or less, Cr: 0.05% or less, Al: 0.02 to 0.04%. , the balance is Fe and unavoidable impurities, and the particle size number is #5 to #9. A first annular wall provided annularly at the end of a hollow tube made of medium carbon steel and a second annular wall provided annularly at the end of a solid stub shaft made of medium carbon steel are friction welded. A method for manufacturing a drive shaft, the method comprising: joining the drive shaft in the axial direction through a portion to obtain a drive shaft;
a forging process of obtaining the solid stub shaft by cold forging;
a friction welding step of friction welding the first annular wall and the second annular wall having an outer diameter of 30 to 50 mm and a wall thickness of 3 to 5 mm;
In the friction welding step, burrs rising from a first inner wall surface, which is an inner wall surface of the first annular wall, and a second inner wall surface, which is an inner wall surface of the second annular wall, respectively rise from the first inner wall surface. It has a burr bottom that is a base end and an orthogonal part along the orthogonal direction of the first inner wall surface, and a connecting radius r that is a radius of curvature of the burr bottom is 0.5 mm or more, and the burr bottom of the burr The root radius R, which is the minimum radius of curvature of the portion between the orthogonal part, is 0.5 mm or more, and the burr bottom angle θ, which is the inclination angle of the burr bottom with respect to the first inner wall surface, is 40° or less, and the burr A method for manufacturing a drive shaft, comprising forming the friction welded portion such that the length L of the burr inclined portion, which is the length along the axial direction of the portion between the bottom portion and the orthogonal portion, is 0.2 to 5 mm. The method for manufacturing a drive shaft according to claim 7,
In the friction welding step, the friction welding portion is formed such that the connecting radius r is 3 mm or less and the root radius R is 2.5 mm or less. The method for manufacturing a drive shaft according to claim 7 or 8,
After the friction welding step,
an annealing step of annealing the drive shaft under conditions of a holding temperature of 650 to 720°C and a holding time of 30 to 90 minutes;
a quenching process of hardening the drive shaft after annealing;
has
A method for manufacturing a drive shaft, wherein NbC, Mo ₂ C, and TiC are precipitated at grain boundaries in the metal structure of the solid stub shaft.